1. Field of the Invention
The present invention relates to a communication channel selection method and a mobile communication apparatus. More particularly, the present invention relates to the improvement of a communication channel selection method of selecting one radio communication channel from a plurality of radio communication channels provided by the system side and relates to the mobile communication apparatus for selecting one radio communication channel from the plurality of radio communication channels provided by the system side.
2. Description of the Background Art
A mobile communication apparatus such as a portable telephone has a channel selection section for selecting a communication channel since the apparatus is generally capable of using a plurality of radio communication channels provided by the communication system, whereby the mobile communication apparatus communicates through a communication channel selected by the channel selection section.
However, all of the communication channels are not always in a usable state. That is, even when a multiplicity of radio communication channels provided by the system side (for example, an earth station) are usable, the radio signal capable of being received by the mobile communication apparatus is limited under a particular circumstance. Accordingly, when determining a communication channel, the channel selection section is required to perform the channel search in a manner that the communication channels capable of being selected are sequentially selected thereby finding a usable communication channel, that is, a channel capable of receiving the radio signal transmitted from the system side.
The communication channel is defined by frequency and time slot in the case of a portable telephone of the TDMA (time division multiple access) system. While, the communication channel is defined by frequency and spreading code in the case of aportable telephone of the CDMA (code division multiple access) system. For example, in the system employing the GSM (global system for mobile communications) system which is one of the TDMA system, several hundred kinds of communication channels can be selected in accordance with combination of the frequencies and the time slots.
Accordingly, in the case where the channel switching is required, communication channels are sequentially selected from a channel list in which selectable communication channels are defined. And, the channel states of the selected communication channels are checked (channel search). When a communication channel in a good channel state is found, this communication channel is acquired. Thereafter, if the channel state of the acquired communication channel is degraded, the same channel search as the aforesaid channel search is executed again. In such a channel search, since communication channels are sequentially selected from the channel list, there is a case where a communication channel in a good channel state can be found through checking only a channel state of one communication channel, but there is also a case where a communication channel in a good channel state can be found for the first time through checking channel states of several hundreds of communication channels.
Although a time required for the checking operation of the channel state differs depending on the communication system, the time required for the channel search becomes in a range of several seconds to several ten seconds at maximum in the case of the communication system requiring a long time for the checking operation. If such a long time is required for the channel search, there arises a problem that a user can not immediately use the radio communication apparatus for communication, even if the user exists within a service area capable of receiving the radio signal from the system.
There has been proposed the portable telephone in which the communication channels within a channel list are grouped and the communication channels of a particular group are subjected to the checking of the channel states thereof in preference to other groups. In such a portable telephone, the channel list includes the communication channels (home channels) provided by the service company with which a user directly contracted and the communication channels (roaming channels) which are provided by other service companies and available for the same portable telephone by the roaming contract or the like, wherein the home channels and the roaming channels are memorized as separate groups. At the time of the channel search, the selection of the communication channel from the group of the home channels is performed in preference to the other groups, and the communication channel is selected from the group of the roaming channels only when none of all the hole channels is in a good channel state. In general, since the channel charge is cheaper in the home channels rather than the roaming channels, this portable telephone is arranged in a manner that the communication channels of the channel list are grouped due to the economical reason. According to such grouping of the communication channels, although the time required for the channel search can be shortened within an area where the home channels can be acquired, it takes a long time for the channel search in an outside area where the home channels can not be acquired since only the roaming channels can be acquired.
Further, there has been proposed the system where a system (for example, an earth station) transmits a radio signal including information as to the communication channels of peripheral cells. For example, in the GSM system, the channel information on the radio signal (a cell corresponding to this radio signal is referred to as a serving cell) transmitted from the earth station monitored by the mobile communication apparatus includes the frequency information on cells (referred to as neighboring cells) adjacent to the serving cell. If the frequency information on such neighboring cells is obtained in advance, even when the mobile communication apparatus moves out of the serving area, the next communication channel can be acquired by the channel search in a short time by performing the channel search only as to such neighboring cells.
One earth station generally provides a plurality of cells adjacent thereto. However, the respective neighboring cells in the GSM system are cells merely adjacent to the current serving cell. In other words, an earth station providing the respective neighboring cells is not always same as an earth station providing the serving cell. Accordingly, an earth station providing a new serving cell selected by the channel search sometimes is different from the earth station having provided the current serving cell.
However, there is a case where a cell provided by the same earth station having provided the current serving cell is selected as a new serving cell as possible. For example, when a mobile communication apparatus locates near the boundary of service areas with different charge systems, it is economical to select a communication channel provided by an earth station with cheaper charge system. Further, for example, when there is an earth station which connects only to a particular network in view of the confidence or the like of the communication, it is required to select a communication channel by discriminating an earth station providing communication channels.
In such a system, it is required at the time of switching communication channels to sequentially extract the information on an earth station, that is, the information as to the charge system with respect to selectable communication channels, thereby determining whether or not an earth station is one providing the service desired by a user. Thus, it is required for each communication channel to extract the service information and determine the condition in addition to the checking operation of the channel state, so that there arises a problem that a time required for the channel search becomes longer by such operations.
Accordingly, the present invention has been made in view of such circumstances and an object of the present invention is to provide a communication channel selecting method and a communication terminal apparatus such as a mobile communication apparatus in which identification information on an earth station are held in advance thereby selecting a communication channel desired by a user in a short time at the time of channel switching.
A communication channel selecting method according to the present invention comprises the steps of: detecting a channel state of a communication channel; extracting from a received signal identification information on a system side communication apparatus which provides the communication channel; storing on a basis of a detection result of the detecting step, the identification information extracted by the extracting step so as to to be link the identification information to the communication channel; and selecting a communication channel on a basis of the identification information stored. According to such an arrangement, at the time of channel switching, a communication channel provided by the same system side communication apparatus can be selected in preference to other communication channels.
A mobile communication apparatus according to the present invention comprises: means for detecting a channel state of a communication channel; means for extracting from a received signal identification information on a system side communication apparatus which provides said communication channel; means for storing on a basis of said detected channel state, said extracted identification information so as to link the extracted identification information to said communication channel; and means for selecting a communication channel on a basis of said identification information stored by said identification information storage means. According to such an arrangement, at the time of channel switching, a communication channel provided by the same system side communication apparatus can be selected in preference to other communication channels.